Mercaptans, or thiols, are sulfur analogs of alcohols. Though similar in molecular structure, the presence of sulfur instead of oxygen elicits many differences. Mercaptans are more odorous, with a stronger and repulsive smell in mercaptans with lower molecular weights. Mercaptans are responsible for a number of wine faults caused by unintended reactions between sulfur and yeast and for the “skunky” odor of beer after it has been exposed to light. Mercaptans are often used as odorants to assist in the detection of natural gas (which is odorless in its pure form).
In addition to differences in organoleptic properties, sulfur is less electronegative than oxygen, thus mercaptans experience weaker intermolecular forces and hydrogen bonding leading to lower boiling points, lower solubility in water or other polar solvents, and higher equilibrium vapor pressure. Mercaptans are also more acidic when compared to their alcohol analogs because sulfur has a softer, more polarizable electron cloud and is capable of distributing the anionic charge deprotonation.
Hydrocarbon fluids, such as crude oil, crude oil emulsions, oilfield condensate, liquefied petroleum gas, natural gas and residual gases and even refined fuels often contain a variety of mercaptans, including thiols of relatively low molecular weight. In the drilling, production, transport, storage, and processing of hydrocarbon stocks, the mercaptans encountered can cause many problems ranging from malodors to metal corrosion. Hydrogen sulfide, carbonyl sulfide and mercaptans are all detrimental sulfur species to hydrocarbon streams when these hydrocarbon streams are being processed.
In liquid hydrocarbon streams, these contaminants, among others, are often found in natural gas liquids, gasoline, diesel fuel, and kerosene. When the liquid hydrocarbon streams are burned, the contaminants can reduce efficiency of catalytic converters and generate corrosion. In gaseous streams, these contaminants are often found in natural gas, process off gas, flash gas, residual gas, biogas, process vapors from storage tanks, transport lines or vessels and associated gas in petroleum production operations. As with liquid streams, when the gas streams are combusted or flared, the contaminants can generate new detrimental sulfur species that can be released into the environment. From both a health/safety and process efficiency perspective, the removal of mercaptans is extremely necessary.
Many methods for removing mercaptans exist. Various additives have been employed for the removal of sulfur compounds from hydrocarbon streams (“sweetening” process). Conventional methods in the prior art for removing mercaptans from hydrocarbons typically involve “sweetening,” wherein mercaptans are oxidized to form disulfides. Light mercaptans (C1-C4) may be removed in an aqueous wash in this process but removal of heavy mercaptans (C4+) is less effective due to the poor water solubility of heavy mercaptans.
U.S. Pat. No. 8,048,175 discloses the use of a mercaptan scavengers having di-substituted azodicarboxylates to remove contaminants from hydrocarbons, including naphthas and gasolines. Oxidation by a strong oxidant reagent such as sodium hypochlorite (US 20120103872) or hydrogen peroxide has been employed. Other methods for mercaptan removal from hydrocarbon streams are also available including oxidation using ozone, biological removal processes, catalytic decomposition and adsorption into solid beds (i.e. functionalized activated carbon), and specialized solvents. Only the two latter methods however are commonly used in the industry.
The most common method utilized today is the reaction with caustic (sodium hydroxide) compounds to form mercaptanate (or thiolate) salts. The reaction with caustic is effective, and sodium hydroxide is readily available at low costs. The caustic treatment however produces a waste stream commonly known as spent caustic that has to be treated or managed properly. The use of caustic for mercaptans removal causes high salt content, and in many cases, there is odor release. The spent caustic can be regenerated by using a catalytic process and oxygen. A regeneration process is shown in FIG. 1. This reaction regenerates the caustic that is re-used in the process; however, it also produces secondary disulfide by-products (also called Red Oil), which are water immiscible materials that can be a challenge for disposal.
For the hydrocarbon producers, transporters, refiners and users, complying with such increasingly stringent specifications has primarily meant using more stringent safety and processing conditions. Amine treating, stripping and scavenging are potential methods for removing these sulfur-containing contaminants online; however, many negative aspects associated with these methods in terms of cost, efficacy and safety render these processes inefficient.
As such, there is a need for improved methods of removing contaminants, especially sulfur-containing contaminants, from both liquid and gas hydrocarbon streams. Preferably, these methods are cost effective, have a small environmental footprint and remove the contaminants, including larger (>C4) mercaptans, without release into the environment.